1. Field of the Invention
This invention relates to a micro-accelerometer made by micromachining techniques and used to detect vibrating waves. The invention is preferably related to a miniaturized accelerometer for detecting earthquake waves.
2. Description of the Prior Art
Individual terminals of a gas supply network must be shut quickly and reliably in the event of large-scaled earthquakes. For this purpose, an earthquake sensor is equipped to each user's terminal of the network to detect earthquakes and to shut the terminal in response to a detection signal.
Existing accelerometers for detecting earthquakes can be classified into two types: one of a broad band type using a small vibrator and capable of detecting vibrations over broad bands and the other of a narrow band type whose frequency characteristics are limited to low frequencies so as to detect only earthquake frequencies. These accelerometers have arrangements in which a mass is connected to a frame via a spring or other resilient member so as to detect acceleration in terms of changes in relative position between the mass and the frame.
In order to implement a low resonant frequency to an existing narrow band accelerometer designed to detect only earthquake waves which have low frequencies, the accelerometer itself must have a large dimension. If acceleration of 100 Gal for the frequency of 1 Hz is to be detected, displacement of the accelerometer as large as 2.5 cm is required. For allowing such a large displacement in a linear system, its vibrator must be, at least, as long as the length of the displacement.
Existing wide band accelerometers have quite high band resonant frequencies, typically 1000 Hz or more, causing the accelerometers to recognize traffic or other vibrations as earthquakes. In order to overcome the problem, they need an electric filter for removing high frequency components from detection signals so as to extract only earthquake signals.
FIG. 12 shows a basic structure of an earthquake sensor currently used in gas meters. The earthquake sensor 100 uses a steel ball 150 housed in a container 110 having a funnel-like cavity 130 in the center of its bottom 120. When an acceleration is applied to the container 110, the steel ball 150 runs up the slope 131 of the cavity 130, and touches an electrode 141 provided on the circumferential wall 140 of the container 110, thus short-circuiting the electrode 141 and another electrode 121 at the bottom, causing a signal indicating application of an acceleration above a predetermined value to be exerted.
This acceleration sensor exhibits the characteristics shown in FIG. 13 indicating acceleration (Gal) on the ordinate and displacement (mm) on the abscissa. That is, when inner diameter of the container 110 is 21.6 mm, diameter of the steel ball 150 is 15.9 mm, and angle .alpha. of the slope of the cavity 130 is 6.42 degrees, the steel ball moves by 2.477 mm at the acceleration of (5/7).multidot.M.multidot.g.multidot.sin.alpha..multidot.cos.alpha. and causes a detection signal to be exerted. In the equation, M is mass of the ball, and g is gravitational acceleration.
An accelerometer used as a seismograph requires sensitivity to accelerations of 85 to 150 Gal for frequencies between 1 and 5 Hz, and requires a roll-off (decrease in sensitivity) of approximately 60 dB per decade above 5 Hz. The general shape of the ideal force-displacement characteristic is shown in FIG. 4 indicating restoring force on the ordinate and displacement on the abscissa. This force-displacement characteristic describes that the spring is very stiff at small accelerations, which causes the mass to move together with the frame, resulting in no relative displacement between the mass and the frame. When the threshold acceleration is reached, causing the threshold force (seismic mass.times.threshold acceleration) to be exceeded, the spring becomes soft, which causes a relative displacement between the frame and the mass.
In order that a detector element does not largely deform when an earthquake acceleration is applied, the detector element must be small, and the use of micromachining techniques to make such accelerometers would be advantageous. In addition, such accelerometers themselves for detecting earthquake waves must have a narrow detection bandwidth of approximately 1 to 5 Hz to eliminate the need for an expensive low pass electric filter.
To meet these requirements, the spring must be stiff before an incoming earthquake acceleration slightly exceeds the system threshold, and must change in characteristics to become soft when the threshold is exceeded. By implementing this feature, the detection bandwidth relative to a large acceleration near the threshold can be narrowed.